The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply, it includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment.
Alternative sources for developing combustible liquid fuel are desirable. Natural gas is an abundant source. The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain appreciable amounts of nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less health and environmental impact than conventional petroleum-based fuels. Fischer-Tropsch synthesis is a preferred means for converting syngas to higher molecular weight hydrocarbon products.
Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C20+ wax, which must be hydroprocessed to provide distillate fuels and other useful products. Often, the wax is hydrocracked to reduce the chain length, and hydrotreated to reduce oxygenates and olefins to paraffins. Although some catalysts have been developed with selectivity for longer chain hydrocarbons, the hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable. The same limitations are observed when hydroprocessing other feeds, such as those derived from crude oil.
Processes have been developed which use a downflow reactor with multiple beds, where a hydrocracking catalyst is present on the first bed or beds, and a milder catalyst is present on one or more beds lying below the hydrocracking catalyst beds. The feed is split into at least a relatively high boiling and a relatively low boiling fraction. The relatively higher boiling fraction is passed through the hydrocracking catalyst beds, and the lower boiling fraction is introduced at one or more different locations in the reactor and passed through the milder catalyst beds. This type of processing is referred to herein as xe2x80x9csplit-feedxe2x80x9d processing. An example of such a process is described in U.S. Pat. No. 5,603,824 to Kyan et al., the contents of which are hereby incorporated by reference, which discloses a process for upgrading a waxy, sulfur-containing hydrocarbon feed mixture.
Fixed bed hydroprocessing reactors used for hydrocracking, hydrotreating, dewaxing and other related processes are often subject to pressure drop build-up through the fixed beds of solid catalyst pellets. This pressure drop build-up can be caused by contaminates found in the feed stream (particulates, rust and scale from piping and other equipment, etc.), from soluble metals originating in crude oil, or from products of undesired side reactions, for example, the reaction of Fe and S to form FeS or from the polymerization of olefinic hydrocarbon molecules. The pressure drop is almost always most pronounced at the top of the reactor, usually in the first of multiple serial beds, where the catalyst particles often act as a crude filter for these materials. In the most severe cases, the contaminates form a crust near the top of the first catalyst bed, eventually requiring plant shutdown to avoid permanent mechanical damage to reactor internals and associated equipment.
The problem is often mitigated by removing the contaminant before entering the hydroprocessing reactor (i.e., by filtration) or by using a number of top-catalyst bed xe2x80x9cgradingxe2x80x9d schemes. One such catalyst grading scheme is described in U.S. Pat. No. 4,615,796 to Kramer, the contents of which are hereby incorporated by reference. A limitation of this approach when used for split-feed hydroprocessing is that only the contaminants in the higher boiling fraction are removed. Solid particulate contaminants present in the lower boiling fraction can foul the lower catalyst beds.
It would be advantageous to provide reactors and methods for hydroprocessing hydrocarbon feedstocks using split-feed hydroprocessing that minimize the pressure drop build-up associated with solid particulate contaminants. The present invention provides such reactors and methods.
The present invention is directed to a method for hydroprocessing hydrocarbon products, preferably Fischer-Tropsch products, and a reactor useful for performing the method. The reactor includes one or more first catalyst beds comprising a catalyst useful for conducting relatively severe hydroprocessing (preferably hydrocracking) and one or more second catalyst beds comprising a catalyst useful for conducting relatively mild hydroprocessing (preferably hydrotreatment and/or hydroisomerization). The second catalyst beds are located at a position in the reactor where they can receive the products from the first catalyst bed(s), at least one of each of the first and second catalyst bed(s) comprises a catalyst grading scheme, and the reactor is set up to receive hydrocarbon feeds at a position above or within the first catalyst bed(s) and above or within the second catalyst bed(s).
A first hydrocarbon feed with a relatively high boiling point is subjected to relatively severe hydroprocessing. A feed comprising 1) the products from the hydroprocessing of the first hydrocarbon feed and 2) a second hydrocarbon feed with a low boiling point which is relatively lower than the first hydrocarbon feed that also includes particulate contaminants is introduced to the second catalyst beds and is subjected to relatively mild hydroprocessing. The catalyst grading scheme minimizes pressure drop build-up through the catalyst beds.
In the invention, a method is provided for hydroprocessing a hydrocarbon feed comprising:
(a) setting up a reactor system which comprises:
(b) one or more first catalyst bed(s) comprising a catalyst useful for conducting relatively severe hydroprocessing on a hydrocarbon feed, and
(c) one or more second catalyst bed(s) comprising a catalyst useful for conducting relatively mild hydroprocessing on a hydrocarbon feed,
wherein the second catalyst bed(s) are located at a position in the reactor where they can receive the products from the first catalyst bed(s), at least one of each of the first and second catalyst bed(s) comprises a catalyst grading scheme, and the reactor is set up to receive hydrocarbon feeds to be hydroprocessed at a position above or within the first catalyst bed(s) and above or within the second catalyst bed(s),
(d) introducing a first hydrocarbon feed with a relatively high boiling point and comprising particulate contaminants to the first catalyst bed(s),
(e) subjecting the first hydrocarbon feed to relatively severe hydroprocessing,
(f) passing a feed blend comprising the products from the hydroprocessing of the first hydrocarbon feed blended with a second hydrocarbon feed having a relatively low boiling point compared with the first hydrocarbon feed, and
(g) subjecting the feed blend to relatively mild hydroprocessing.
The hydrocarbon feeds are preferably derived, in whole or in part, from Fischer-Tropsch synthesis, although they can include products from petroleum refining or other suitable sources. In one embodiment, the relatively high boiling fraction has a normal boiling point of greater than about 650xc2x0 F. and is composed predominantly of C20+ components and the relatively low boiling fraction has a normal boiling point below about 700xc2x0 F. and is composed predominantly of C5-20 components.